The present invention relates generally to biological safety cabinets.
Biological safety cabinets are laboratory containment devices equipped with High Energy Particulate Air (HEPA) filters. These cabinets are used in microbiological laboratories and provide a work area with safe environment in which a variety of experiments and studies can be performed. Rather than providing only a hood above a working surface, these cabinets provide a more protective working environment. The safety cabinet has a frame that surrounds the work area on all but one side. The remaining open side is enclosed by a moveable sash. The sash may be moved upwardly to provide access to the work area, so that work can be performed. The sash may be moved downwardly to partially or completely close the work area. A blower unit is provided in the cabinet above the work area. The blower is used to circulate air downwardly through the safety cabinet. A portion of this downward air flow forms an air curtain at the front of the cabinet work area and passes beneath the floor of the work area and a portion is directed to the back of the cabinet where it is drawn upwardly through a plenum chamber. This air may be contaminated by materials being used within the working environment. Therefore, prior to being exhausted into the room or a fume system, the air is first passed through a HEPA exhaust filter.
The blower is operated so there is sufficient air flow through the work area to insure that any harmful materials are contained and eventually passed to a filter area rather than escaping into the room or exhausted into the atmosphere. To this end some air is drawn into the safety cabinet about the open perimeter formed when the sash is in an open or partially open position.
The prior art safety cabinets are typically provided with a sash grill located below the bottom of the sash. This sash grill forms the lower-most surface of the opening into the work area. Typically, the sash grill is provided with a number of perforations, through which air can flow. Air flows downwardly from the blower along the back of the sash and into these perforations. Air is also drawn inwardly from the exterior of the cabinet along the surface of the sash grill and into the perforations. The air flowing through the sash grill flows under the work surface and upwardly through the plenum at the back of the cabinet to be recirculated or exhausted.
Safety cabinets have heretofore utilized a sash grill having a generally flat surface which gives rise to a number of disadvantages. The flat surface may be used by those operating the safety cabinet as a surface on which to place a variety of labware. This is undesirable because objects located on the sash grill present a source of possible contamination of the room, and may be inadvertently broken if bumped or knocked onto the floor. Moreover, by placing an object on the sash grill, a portion of the perforations therein may be blocked, which can adversely affect the air flow of the safety cabinet. The flat surface of the sash grill also results in a large portion of the perforations therein becoming blocked by a user's arm as the user performs work within the safety cabinet. As the user's arm blocks the perforations in this fashion, it is difficult to properly maintain the negative pressure environment about the user's arm, thereby risking possible contamination. The flat sash grills of the prior art also present a right angle with the work surface which projects far enough above the work surface that labware is sometimes broken when it bumps against the projecting vertical face. It is thus desirable to provide a sash grill which does not provide a flat surface and does not present a right angle corner at the entrance to the work area opening.
Another drawback of prior art sash grills is attributable to the fact that the grills are formed with a front face that is at a right angle to the flat top of the grill. This orthogonal relationship results in an air flow that is less than desirable. When air is drawn inwardly and through the perforations in the sash foil, it may cause a turbulence in the air flowing downwardly along the back of the sash and through the working environment. This turbulence is increased by the right angle relationship, as the air encountering the front face of the grill will be partially directed upwardly over the front face before being drawn through the perforations in the flat top of the grill. Therefore, a biological safety cabinet is needed with a sash grill that improves the air flow and safety of the cabinet.
Similarly, air may be drawn into the opening of the safety cabinet along the sides of the cabinet adjacent the opening when the sash is in an open or partially open position. In prior art safety cabinets, the front sides of the cabinet are oriented at right angles relative to the interior side walls. When air is drawn into the cabinet along these sides, it will initially be directed away from the interior surface of the interior walls. However, it is much more desirable to cleanly “sweep” the interior walls of the cabinet, to ensure the best possible containment of any harmful materials. A biological safety cabinet having a construction that draws air inwardly to cleanly sweep the interior side walls is needed.
After the safety cabinets have been used for a certain period of time, they must be decontaminated. One method for performing this decontamination involves sealing the front of the safety cabinet with a plastic sheet. When the prior art safety cabinets are being decontaminated, it is often necessary to first remove the sash to insure proper decontamination. This is attributable to the location of the sash within a U-shaped channel where contaminants may accumulate. This procedure is time consuming and risks damage to the sash. If the sash is dropped it may shatter, and contaminate an entire room. Thus, a biological safety cabinet which can be decontaminated without removal of the sash is needed.
Another drawback of prior art safety cabinets involves the lower edge or handle of the moveable sash. When the sash is in an open or partially open position, two bodies of air are coming together adjacent the handle of the sash. One body of air is flowing from the exterior of the cabinet into the interior thereof. The second body of air is flowing downwardly from the blower unit of the safety cabinet along the back of the sash. In prior art cabinets, the sash handle has transitioned from the front face to the bottom face at a right angle. This results in the inwardly flowing air meeting the downwardly flowing air at a right angle, causing turbulence. As noted above, turbulent air flow adjacent the opening of the cabinet is undesirable. A sash handle that reduces turbulence would represent an improvement over the prior art.
As stated above, the biological safety cabinet is operated with the benefit of a blower which provides an air flow so that harmful materials are contained within the cabinet. The cabinets are constructed with the blower above the working environment, and the working environment is subject to a continual flow of air to contain contaminants and then move them to a filter area. Above the working environment and beneath the blower, is a supply filter and a positive pressure plenum. The pressure plenum receives air from the blower and directs it through the supply filter.
To monitor the pressure within the cabinet, prior art safety units have used a pressure gauge mounted on the exterior of the cabinet, with the pressure being monitored in the positive pressure environment of the pressure plenum immediately below the blower. Monitoring the positive pressure allows a more meaningful pressure reading to be obtained and used by the laboratory personnel. However, the air within the pressure plenum immediately below the blower has not yet been filtered. As such, the air may contain harmful materials from the working environment below. If the gauge on the exterior of the cabinet were to leak, contaminated air would be allowed into the room. In some instances this concern has been addressed by placing a HEPA filter in the pressure line to the readout gauge. This of course results in additional expense both initially and for ongoing maintenance. Another method of addressing the potential problem of contamination through the pressure gauge has been to monitor the air pressure in a negative pressure environment (relative to the atmosphere surrounding the cabinet) thus eliminating the possibility of contamination as a result of leakage through the gauge into the room. Monitoring and displaying a negative pressure, however, is more difficult to translate into meaningful and usable numbers by laboratory personnel. A monitoring apparatus is therefore needed which does not require any additional filters and allows the monitoring and display of a positive pressure, while eliminating the risk of possible contamination of the room environment.
It has been found that it is desirable to equip the safety cabinet with a “towel catch” to catch or filter out large objects from the returning air flow prior to being recirculated through the blower. This towel catch removes such things as paper towels and small laboratory items from the returning air stream. Prior art safety cabinets have located this towel catch in the plenum formed by the rear wall of the work area and the rear wall of the safety cabinet. While this location is effective in removal of the desired items, it is impossible to visually inspect without taking the cabinet apart. One method typically utilized for inspecting these prior art towel catchers is to reach up within the plenum and feel the towel catch to determine if any paper towels or other objects are lodged within or against the towel catcher. This method can be uncomfortable and dangerous to the extent that pieces of broken laboratory glass and other sharp objects may be lodged within the towel catch. The towel catch itself is normally formed from metal with sharp edges which presents a safety hazard in and of itself if it is placed in a traditional location where it is not visible to a worker cleaning it. Therefore, a towel catch that is readily accessible and can be visually inspected is needed.
Another drawback of prior art safety cabinets involves the construction of the sash. The sash of the safety cabinet is moveable upwardly and downwardly, to allow better access to the working environment when needed and to more fully enclose the working environment when access is no longer needed. In prior art safety cabinets, the rear of the sash is provided with a seal to prevent any contaminated air from escaping the working environment. The seal wipes the back of the sash as the sash is raised. This arrangement is disadvantageous in that the wiping action may create an aerosol containing contaminants from the rear of the sash. While in other prior art constructions holes communicating with the exhaust system have been utilized in place of seals, such constructions have not been particularly effective, largely because there has been no means for insuring a uniform negative pressure across the exhaust holes. Thus, an arrangement is needed for a biological safety cabinet that eliminates the need for a wiping seal at the rear of the sash and instead provides for a uniform negative pressure which will insure removal of any contaminated air from the back side of the sash.
Yet another drawback of existing prior art safety cabinets involves the design of the positive pressure plenum box. This box is located in the area below the blower and above the work area. More specifically, in prior art cabinets, air leaving the blower is directed to a perforated plate and then through a supply filter prior to be recirculated downwardly through the work area. The perforated plate is used to more evenly distribute the air flow over and through the supply filter. The perforated plate creates an undesirable increased load on the blower and can interfere with the function of the supply filter. Moreover, this prior art construction does not distribute air across the supply filter as evenly as desired. Therefore, a structure is needed that both evenly distributes the flow over and across the supply filter while not overly increasing the load on the blower or interfering with the function of the supply filter.
Prior art safety cabinets are typically equipped with exhaust control systems. As contaminated air passes through the blower of the safety cabinet, some of the air is recirculated through the supply filter as described above and some of the air is routed through an exhaust filter. This exhaust air is either discharged into the room, or it passed to an exhaust system associated with the safety cabinet which moves the air out of the building. In cabinets routing the exhaust air directly back into the room, the prior art cabinets have merely routed the air directly upwardly. Prior art units routing the air into a building exhaust system direct typically employ duct work coupling the safety cabinet exhaust to the building exhaust system. Both prior art embodiments require a certain amount of additional space above the ceiling of the safety cabinet to allow for the exhaust control systems. This need for space can place limitations on the rooms in which the safety cabinets can be used.
In addition to routing the exhaust air, the exhaust control systems of the safety cabinets are used to balance the air flow through the safety cabinet. Prior art exhaust control systems use a guillotine damper to allow more or less air to be exhausted, as needed to balance the air flow through the safety cabinet and achieve the proper pressure within the cabinet. This damper places some additional load on the blower by restricting air flow to the filter. Furthermore, a damper is not aerodynamically efficient and interferes with the uniform flow of air. Such dampers are normally not readily accessible for making adjustments. The use of such a damper also tends to cause air to flow unevenly through the filter thus not effectively using the entire filter surface area. Therefore, a more efficient exhaust control system is needed for a biological safety cabinet that reduces undesired blower loading, makes better utilization of available filter surface area and is readily accessible.